Currently, 3rd generation cellular communication systems are being rolled out to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users operating on the same carrier frequency and using the same time intervals. This is in contrast to time division multiple access (TDMA) systems, where user separation is achieved by assigning different time slots to different users.
An example of a CDMA communication system is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of UMTS, can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a conventional cellular communication system, cells in close proximity to each other are allocated non-overlapping transmission resources. For example, in a CDMA network, cells within close proximity to each other are, despite commonly being allocated the same carrier frequency, allocated distinct spreading codes (to be used in both an uplink communication direction and a downlink communication direction). This may be achieved by, for example, employing the same spreading codes at each cell, but a different cell specific scrambling code. The combination of these leads to effectively distinct spreading codes at each cell.
A technique for increasing the unicast capacity of a cellular network, i.e. the number of simultaneously supportable users, and/or effectively increasing the transmission power is to take an existing cellular base station with an omni-directional coverage region, and split this into a number of independent sectors. In such a situation, each of these new sectors becomes a separate cell, with its own transmitter, a directional antenna and a reduced coverage area. Such a network is referred to as a sectorised cellular network.
In order to provide enhanced communication, the 3rd generation cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service is multimedia. The demand for multimedia services that can be received via mobile phones and other handheld devices has been growing rapidly over the last few years. However, multimedia services, due to the nature of the data content that is to be communicated, require a high bandwidth.
As radio spectrum is at a premium, spectrally efficient transmission techniques are required in order to provide users with as many broadcast services as possible, thereby providing mobile phone users (subscribers) with the widest choice of services. One such spectrally efficient transmission technique in the provision of multimedia services is to ‘broadcast’ some multimedia signals, as opposed to sending the multimedia signals in an uni-cast (i.e. point-to-point) manner. With such transmission techniques, typically tens of channels carrying say, news, movies, sports, etc. may be broadcast simultaneously over a communications network.
Broadcast services are conventionally transmitted from a network of transmitters and repeaters (similar to conventional terrestrial Television/Radio transmissions). These transmitters and repeaters typically comprise high antenna masts with omni-directional antennas and high power transmitters. Thus, the coverage area of each transmitter in such a system is typically very large. This scenario encompasses modern digital broadcasting technologies targeted at moving handheld wireless subscriber communication units, such as DVB-H.
Broadcast services may also be carried over cellular networks, although conventionally only unicast user traffic is carried over a cellular network. However, delivering broadcast services over cellular networks is extremely attractive, for at least the following reasons:
(i) It provides a self-contained communication system i.e. the signalling required (e.g. for encryption key exchange) and uplink data (e.g. to support interactive services) can be carried by the same system that is used to deliver the broadcast service.
(ii) It allows a cellular operator to reuse existing infrastructure to provide these new services.
(iii) It allows a cellular operator to use spectrum that they already own.
Technologies for delivering multimedia broadcast services over cellular systems, such as the Mobile Broadcast and Multicast Service (MBMS) for UMTS, have been developed over the past few years. In these broadcast cellular systems, the same broadcast signal is transmitted over non-overlapping physical resources on adjacent cells within a conventional cellular system, for instance using effectively different spreading codes in a CDMA system. Consequently, at the wireless subscriber unit, the receiver must be able to detect the broadcast signal from the cell that it is connected to. Notably, this detection often needs to be made in the presence of additional, potentially interfering broadcast signals, transmitted on the non-overlapping physical resources of adjacent cells.
Enhancements to broadcast transmissions may be achieved by employing what is known as a single frequency network (SFN) approach. Here, the same broadcast signal is transmitted over identical physical resources on adjacent cells of a cellular network. These identical physical resources may include, for example, carrier frequency, CDMA spreading code, timeslot, etc. In this manner, transmissions from adjacent cells of the cellular network are seen as multi-path components of the same signal being received at the receiving unit, instead of potentially interfering signals as described above. Thus, a reduction in interference at the subscriber unit receiver may be observed, by treating the received signals as multi-path components of the same signal, which enables higher data throughput rates to be delivered.
However, the inventors have identified that broadcast transmissions in a synchronised, sectorised cellular network may be suboptimal, for example in a synchronised, sectorised cellular network that employs an SFN approach i.e. where all cells transmit identical signals (SFN) at identical time instants. In this regard, if the cells are identically located (e.g. where a sectorised base station site is used) then received signals from the different sectors are combined in a time-synchronous manner. At a receiving unit the signals from different sectors of the same base site will appear together (at the same time).
Here, multiple non-faded signals with random phase are known to combine to produce a faded signal. Thus, by employing SFN in a synchronous, sectorised broadcast network, there is the potential to turn a previously benign propagation channel (e.g. one that exhibits non-faded channel conditions) into a harsh Rayleigh faded propagation channel. This has a resulting detrimental effect upon the sustainable data throughput rate.
Thus, there exists a need for an improved cellular communication system, communication unit and method of broadcasting that may alleviate one or more of the aforementioned problems.